Conventionally, a method for producing copper wiring by applying a copper foil over a substrate and etching the same has been used predominantly. However, this producing method, using etching, has a problem that a large amount of liquids and materials are wasted.
Then, the following method for producing a wiring board has been known as a method that does not use etching; a paste-form conductive composition containing metal (e.g. silver, copper, etc.) particles having a particle diameter of a micron order and an adhesive (e.g. epoxy-based adhesive, acrylic adhesive, silicone-based adhesive, etc.) is applied over a substrate, and is heated at 150° C. to 180° C. (see, for example, Non-Patent Document 1). By this producing method, distances between metal particles in the conductive paste are decreased when the adhesive is heated and hardened, and consequently the metal particles become dense and allow current to pass therethrough, whereby wiring is produced. With this producing method, however, an electric resistance obtained is about 5×10−5 Ωcm, which is relatively high for practical application, and therefore, a lower electric resistance has been demanded.
Another method has been known also, in which a paste-form conductive composition obtained by dispersing microparticles of a silver compound such as silver oxide in a reducing organic solvent is applied over a substrate, and is heated at the vicinity of 200° C., whereby wiring is produced (see, for example, Patent Document 1). By this producing method, microparticles of a silver compound such as silver oxide in the paste change to silver particles when the composition is heated at the vicinity of 200° C., and consequently the silver particles are connected and allow current to pass therethrough, whereby wiring is produced. However, this producing method has the following problems: since this method involves a quantitative reduction reaction of the microparticles of the silver compound such as silver oxide, an intense reaction with the reducing organic solvent occurs, and owing to a large amount of gas generated, such as a gas generated by the decomposition of the reducing organic solvent and oxygen gas generated by the reduction of the silver compound, irregular voids are formed in the conductive composition, which become stress concentration points that make the conductive composition easily destroyed and dangerous upon handling. A method modified in such a manner that silver particles of a micron order are mixed in the composition in order to solve these problems has been known also, but this merely provides a slight improvement, though the degree may vary, since the producing method is based on, as the principle, the metal connection caused by the reduction of microparticles of a silver compound such as silver oxide.
Further, a conductive composition containing silver oxide microparticles and a reducing agent that reduces the same has been known (see, for example, Patent Document 2). This conductive composition also has a problem in that a high-temperature reaction heat is generated, which causes a gas to be generated, as in the above-described case.
A granular silver compound with an organic compound having 1 to 8 carbon atoms being adhered to surfaces of particles has been known (see, for example, Patent Document 3). When this silver compound is heated, the organic compound on the surfaces act as a reducing agent, and as a result, the granular silver compound can be reduced to silver. However, this granular silver compound also has a problem in that a high-temperature reaction heat is generated, which causes a gas to be generated, as in the above-described case.
A conductive paste composed of silver, silver oxide, and an organic compound having a property of reducing silver oxide has been known (see, for example, Patent Document 4). This conductive paste also has a problem in that a high-temperature reaction heat is generated, which causes a gas to be generated, as in the above-described case.
A method for producing a conductive material has been known, in which a porous conductive material having a voidage of 20% to 60% and having a content of an organic substance of 20% or less with respect to the mass thereof, which is obtained by heating a composition composed of silver oxide (I) Ag2O so as to change the silver oxide into silver, is subjected to plating additionally (see, for example, Patent Document 5).
Another method also has been known, in which a paste-form conductive composition containing a low crystallized silver filler having a particle diameter of a micron order and silver nanoparticles is applied over a substrate, and is heated at the vicinity of 200° C., whereby wiring is produced (see, for example, Patent Document 6). By this producing method, when the foregoing composition is heated at the vicinity of 200° C., the silver nanoparticles are molten or sintered, and fused so as to adhere to one another, and allow current to pass therethrough, whereby wiring is produced. In this producing method, however, there is a problem that the silver nanoparticles cost high.
In the case of the above-described producing methods, it is necessary to use an adhesive that makes it difficult to decrease an electric resistance, to use microparticles of a silver compound as a principal material, such as silver oxide being unstable and having a strong reducing tendency, or to use a conductive composition containing expensive silver nanoparticles.
In the case that such a material of a conventional technique is applied to electronic components as a bonding material for device electrodes, die attaches, and microbumps, this material applied, for example, in a light-emitting device is used for mounting light-emitting elements on a substrate such as a lead frame or a printed circuit board. Light-emitting elements in recent years have a problem in that an adhesive discolors owing to heat generated by the application of high current, and an electric resistance varies with time as an organic component of a resin or the like is degraded by heat and light. Particularly in the case of the method in which the bonding completely depends on the adhesion power of the adhesive, it is concerned that there might occur the following critical problem: when an electronic component is mounted by soldering, the bonding material may lose an adhesion power under the solder melting temperature, and separation occurs, which results in failure of lighting.    Patent Document 1: JP 2003-309352 A    Patent Document 2: JP 2004-253251 A    Patent Document 3: JP 2005-200604 A    Patent Document 4: JP 2005-267900 A    Patent Document 5: JP 2006-24808 A    Patent Document 6: 2005-129303 A    Non-Patent Document 1: Yi Li, C. P. Wong, “Recent advances of conductive adhesives as a lead-free alternative in electronic packaging: Materials, processing, reliability and applications”, Materials Science and Engineering, 2006, R 51, pp. 1-35.